Population growth is the change in population over a period of time. Population size is affected by birth rates, death rates, emigration and immigration. Population growth from 1800-2011 was from 1 billion to 7 billion. In 2011, it was estimated that around 90 million people added to the existing population.
Exponential population growth- This is when birth rate is constant over time and is not limited by disease or food. In exponential growth, only the birth rate controls the population growth.
Logistics population growth- When a population shows exponential growth, the members are not limited by disease or food. The rate of population growth in logistics is proportional to the amount but it is obstructed by limited resources and competition. The rate of growth is fast initially, and slows down afterwards since there are many individuals competing on limited resources and space. When the population reaches equilibrium, the growth rate becomes zero, the size of the population remaining constant if there are no interruptions.
The human population growth is at the exponential phase. The birth rate has been relatively constant over a period of time and it is not limited by disease or food. The human population began slowly, but over the years, from generation to generations, the growth rate increased rapidly to its present state. The earth is therefore in serious danger of being overpopulated by humans. The resources are eventually used up, the carrying capacity of the earth is exceeded and the environment is subjected to monumental degradation.
http://www.cliffsnotes.com/study_guide/Population-and-Demographic-Variables.topicArticleId-26957,articleId-26943.html
